1. Field of the Invention
The present invention relates to a method for anodizing a conductive film such as a metal film formed on a substrate and an apparatus for anodizing the same using this method.
2. Description of the Related Art
In a thin-film element such as a thin-film transistor, a multilayer wiring board, or the like, the withstand voltage between a lower metal film (a lower electrode and a lower wiring layer) and an upper metal film (an upper electrode and an upper wiring layer) interposing an insulation film is considerably increased to prevent a short circuit between the lower and upper films. The lower metal film is thus anodized to form an oxide film on the surface thereof.
The anodic treatment of a lower metal film (a lower electrode and a lower wiring layer) is generally carried out by soaking a substrate (e.g., a glass substrate) on which the lower metal film is formed, in an electrolytic solution so as to oppose the lower metal film to a cathode, and then applying a voltage between them.
When the voltage is applied between the metal film and its opposing negative electrode (cathode) in the electrolytic solution, the metal film serving as a positive electrode (anode), is reacted therein and starts to be anodized from its surface, thereby forming an oxide film thereon. The thickness of the oxide film can arbitrarily be determined by controlling the voltage applied between both the negative and positive electrodes.
In the above-described anodic treatment, the composition of the electrolytic solution slightly varies as time passes, and the quality of the oxide film varies accordingly.
Conventionally, though the anodic treatment is performed while maintaining the fixed concentration of the electrolytic solution, the quality of the oxide film is degraded as time passes.
If a lower metal film of a thin-film element, a multilayer wiring board or the like is anodized by the conventional anodizing method, an oxide film portion formed on the surface of the metal film anodized early has a considerably high acid-resistance, but an oxide film portion formed on that of the metal film anodized late has a low resistance to strong acid such as BHF (buffered hydrofluoric acid).
Since the oxide film has a low resistance to the BHF, it is damaged by etching using the BHF in the subsequent manufacturing step, and a short circuit may be caused between the lower and upper metal films.
As is well-known, in a reverse-stagger thin-film transistor, a blocking insulation film is formed on a channel region of an I-type semiconductor layer. This blocking insulation film is formed to prevent the channel region of the I-type semiconductor layer from being damaged by etching of the surface of the I-type semiconductor layer when a portion of an N-type semiconductor layer formed on the I-type semiconductor layer located between the source and drain electrodes is removed by etching. In general, the blocking insulation film is formed of SiN (silicon nitride) which is the same material as that of a gate insulation film.
The blocking insulation film is formed by forming an SiN film and then patterning it using BHF as an etchant by photolithography. Since the I-type semiconductor layer of a-Si (amorphous silicon) formed under the blocking insulation film is usually generated with pinholes, the etchant passes through the pinholes to etch the gate insulation film (SiN film) formed under the I-type semiconductor layer, with the result that pinholes are formed in the gate insulation film, too. Therefore, the surfaces of a gate electrode and a gate wiring layer, which are formed under the gate insulation film, are exposed to the etchant (BHF) passing through the pinholes of the gate insulation film.
If an oxide film formed on the surfaces of the gate electrode and gate wiring layers is very resistant to strong acid such as BHF, the oxide film is not damaged.
Since, however, the oxide film portion formed on the surface of the gate electrode and the gate wiring layer have a low resistance to the strong acid such as BHF, if such an oxide film is exposed to the etchant such as BHF, it is damaged to cause defects such as pinholes.
If the defects such as pinholes are present on the oxide film formed on the surfaces of the gate electrode and gate wiring layer of the thin-film transistor, the withstand voltage characteristics of the oxide film are deteriorated, with the result that a short circuit occurs between the gate electrode (gate wiring layer) and the source and drain electrodes (data wiring layer).
The generation of the short circuit is not only limited to the thin film transistor, but is true of other thin-film element such as a thin-film diode, a multilayer wiring board, and the like, in which an oxide film on the surface of a lower metal film, which is anodized late, is damaged by etching an insulation film (SiN film) using strong acid such as BHF to cause a short circuit between the lower and upper metal films.
The thin-film element, the multilayer wiring board, and the like having an oxide film formed on the lower metal film using the conventional anodizing method, have a drawback in which the rate of occurrence of short circuits differs from time to time when the lower metal film is anodized, and the manufacturing yield is decreased accordingly.